In video coding systems, spatial and temporal redundancy is exploited using spatial and temporal prediction to reduce the information to be transmitted. The spatial and temporal prediction utilizes decoded pixels from the same picture and reference pictures respectively to form prediction for current pixels to be coded. In a conventional coding system, side information associated with spatial and temporal prediction may have to be transmitted, which will take up some bandwidth of the compressed video data. The transmission of motion vectors for temporal prediction may require a noticeable portion of the compressed video data, particularly in low-bitrate applications. To further reduce the bitrate associated with motion vectors, a technique called Motion Vector Prediction (MVP) has been used in the field of video coding in recent years. The MVP technique exploits the statistic redundancy among neighboring motion vectors spatially and temporally.
When MVP is used, a predictor for the current motion vector is chosen and the motion vector residue is transmitted instead of the motion vector itself to conserve bitrate associated with motion vector transmission. The MVP scheme can be applied in a closed-loop arrangement where the predictor can be derived at the decoder based on decoded information and no side information has to be transmitted. Alternatively, side information can be transmitted explicitly in the bitstream to inform the decoder regarding the type of motion vector predictor selected. While MVP can be used for inter-coded blocks to conserve bandwidth, it can also be used for SKIP and DIRECT coded blocks to substantially reduce bitrate for the underlying blocks. In conventional temporal MVP, the predictor is often based on a single candidate such as the co-located motion vector in the previous frame/picture. If the co-located motion vector in the previous frame/picture does not exist, the predictor for the current block is not available. It is desirable to improve the performance of MVP so as to reduce the bitrate of the coding system. The improvement may be achieved by designing better MVP that can provide a more accurate prediction and improve the availability of the predictor. Furthermore, it is desirable that the MVP can be operated in a closed-loop fashion so that no side information or minimum side information is required.